As technology development and demand for mobile devices increase, demand for secondary batteries as energy sources is sharply increasing, and among the secondary batteries, a lithium secondary battery that has high energy density and voltage is commercialized.
Polyvinylidene fluoride (PVDF), which is currently being widely used as a binder of positive and negative electrodes, is a polymer resin dissolved in an organic solvent such as N-methyl-2-pyrrolidone (NMP). Although PVDF is not an adhesive originally, PVDF is widely used as a binder of an electrode active material because PVDF has good miscibility with a graphite material, and an electrode plate having high adhesion can be fabricated by adding PVDF at an amount of about 8 to 10% of graphite.
However, because PVDF covers an active material in a state in which, for example, polymer fibers are filled, PVDF deteriorates battery performance inherent in an electrode active material in terms of capacity and efficiency. Also, because PVDF has insufficient flexibility, a bond tends to be broken and a cycle characteristic is likely to be deteriorated when a material having a large specific surface area and high expansion/contraction rate during charging and discharging such as natural graphite or a metal-based active material is used as an electrode active material.
Because PVDF, which is an existing solvent-based binder, causes the above problems, the use of styrene-butadiene-based rubber (SBR) has been recently studied, and the SBR is currently being commercially used in various ways.
However, although the SBR binder has advantages of being environmentally friendly and capable of increasing battery capacity, the SBR binder is not that effective in terms of adhesion itself despite an improvement in adhesion durability due to elasticity of a rubber. Particularly, although reducing a use amount of a binder is required to satisfy high capacity and high output of a secondary battery, there is a limitation in reducing the use amount with a current adhesion level of the SBR binder. For example, when a content of the binder in an electrode is insufficient, adhesion with a negative electrode current collector may be reduced, and thus a probability that cutting or deintercalation of the electrode may occur during charging and discharging in a secondary battery may increase. Also, because adhesion between a negative electrode current collector and an active material may be lowered than cohesion between active materials due to migration of the SBR binder during fabrication of a negative electrode, SBR should be used at an amount larger than an actually required amount to secure minimum adhesion between the negative electrode current collector and the active material.
Consequently, there is a growing need for development of a negative electrode in which resistance in the electrode is decreased due to an increase in adhesion between a negative electrode current collector and an active material and cohesion between active materials while a content of the SBR binder is reduced, thereby being capable of improving the performance of a secondary battery.